Screw propellers



- Feb. 26, 1963 \l WITNESSES INVENTOR.

G. c. ENGSTRAND 3,078,925

SCREW PROPELLERS Filed June 11, 1962 United States Patent 3,078,925 SCREW PROPELLERS Gunner C. Engstrand, Bellmore, NY. (44 Whitehall St., New York 4, N.Y.) Filed June 11, 1962, er. No. 201,4il9 2 Claims. (Cl. 170-459) My invention discloses a marine propeller of extreme eiliciency and power. Marine propellers are generally so inefficient that the insertion of nozzles, deviating plates etc., enhances their performance considerably. The particular object of my invention is to utilize the whole inertia resistance of the propeller race without setting up a spinning motion therein. I achieve my object by using propeller blades made up of cylindrical blade segments having rhomboid cross section, and having the developed section widths, from the tip to the hub substantially equal to the pitch of the propeller divided first by the numbers of blades and then again divided by the cosine value of the inclination angle of the segment with the disc area of the propeller.

In the drawing:

FIGURE 1 is a plan view of the propeller.

FIGURE 2 is a side view of the propeller mounted on a shaft, and provided with a conventional fair-water.

FIGURE 3 is a diagram showing a developed cylindrical blade segment and the underlying slip flow theory of the propeller action.

In the drawing where like reference characters denote corresponding parts, 1 is my preferred screw propeller, which comprises the hub 3 on which preferably three blades are mounted. The hub 3 is attached to the shaft 4 in the conventional manner and it is preferably provided with a fair-water 5. The back and face sides of the blades are parallel with the generating lines 6, as shown on the drawing, and the apparent pitch of the propeller is therefore constant for the entire length of the blades 2. FIGURE 3 shows the Water flow over a typical developed cylindrical blade section 7 of my propeller.

In order to truly understand the propeller action, you must realize that all acceleration changes that cause pressures on the active blade surface must take place close to the rotating blade surfaces on which they act. The propeller problem, considered from this point of view, becomes simple and high school mathematics will suffice for its complete quantitative solution. Fundamentally, the screw propeller is a rotating plane that is being continuously pressed straight backwards by the ships resistance. Therefore, when the propeller blade cuts through the water and sustains a steady load, sharply defined influence regions appear on the two sides of the blade, within which regions all acceleration changes occur that cause the propeller thrust. These regions are triangular shaped having the leading edge as apex and axisparallel through the trailing edge as base on each side of the blades, all as shown in FIGURE 3.

Realizing that the intensity of thrust depends upon the speed with which the Water is renewed inside these acceleration regions, it becomes evident that the water acted on in one revolution equals a column the length of which equals the slip for one revolution being accelerated backwards with an acceleration equal to the pitch of the propeller. Further, considering that the pressures must be vertical to the active blade surfaces,

"ice

there must also exist forces which do not add or detract from the required forward reaction, i.e., there must be directed across the race and form together with the forward reaction a resultant that is vertical to the blade surface.

Referring back to FIGURE 3, it is evident that the amount of water acted on by the blade elements equals S X cos aX'IrD/ cos a, and as the acceleration of this quantity of water must be at right angles to the blade surface the acceleration must equal the pitch divided by cos a.

When integrating all the cylindrical sections the propeller thrust becomes where r.p.s. indicates revolutions per second, 64 the weight in pounds of one cubic foot of salt water, 32 the acceleration of gravity in ft. per second and S, P, and D, the slip in feet per revolution, P the pitch and D the diameter in feet. Inasmuch as the propeller thrust is also proportional to the blade area it is clear that the total blade width at any radius of the cylindrical blade sections must equal the acceleration vertical to the blade surface, or the pitch divided by cos a.

Finally, the maximum propeller thrust occurs when the pressure on the suction side is zero and the pressure on the face side is substantially 15 pounds per square inch of the projected blade area, and therefore in my propeller with a hub diameter of one fifth of the propeller diameter, maximum propeller thrust becomes P .4D l5 144 pounds with P and D given in feet. For the most eiiicient propeller the integrated propeller thrust above given must equal the maximum thrust value, because at that point the propeller diameter becomes minimum with a minimum friction loss, and by making the blade edges to conform with the boat speed, the thickness factor is practically eliminated, and the described propeller is therefore the most efficient propeller that it is possible to construct by mechanical means.

What I claim is:

l. A propeller having its blades composed of substantially rhomboid cylindrical sections, the apparent pitch of the back and the face surfaces of the blades being constant for the entire blade length, and the width of said cylindrical sections varying inversely with the cosine value of their inclination angles with the disc area of the propeller.

2. A propeller having its blades composed of substantially rhomboid cylindrical sections, the apparent pitch of the back and the face surfaces of the blades being constant for the entire blade length, and the width of said cylindrical sections varying inversely with the cosine value of their inclination angles with the disc area of the propeller, and having the apparent pitches of the leading and trailing edges constant for the entire blade length and substantially conforming with the speed of the vessel.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A PROPELLER HAVING ITS BLADES COMPOSED OF SUBSTANTIALLY RHOMBOID CYLINDRICAL SECTIONS, THE APPARENT PITCH OF THE BACK AND THE FACE SURFACES OF THE BLADES BEING CONSTANT FOR THE ENTIRE BLADE LENGTH, AND THE WIDTH OF 